Cancer cells frequently and perhaps invariably possess aberrations in the genetic pathway of programmed cell death. The BCL-2 family of proteins plays a critical role in the signaling and execution of death signals. A common mechanism by which cancer cells evade programmed cell death is by altering the ratio of antiapoptotic/ pro-apoptotic BCL-2 members, resulting in a functional excess of BCL-2 antiapoptotic molecules. This proposal describes strategies to test the hypotheses that (a) anti-apoptotic BCL-2 family members can be specifically targeted, and (b) BCL-2 is a valid target for anti-cancer therapy. To address (a), binding interactions between anti-apoptotic BCL-2 family members and BH3 domains from "BH3-only" family members will be examined using fluorescence polarization to determine the specificity of interaction between the two classes of proteins. The relevance of the binding interactions will be validated using functional mitochondrial and cellular assays of apoptosis. From these studies will emerge oligo-peptides which function as prototype inhibitors of anti-apoptotic BCL-2 family members. This approach has already shown promise in the preliminary characterization of a peptide inhibitor of BCL-2 based on the BH3 domain of BAD. To address (b), a mouse model of leukemia which is dependent on BCL-2 for maintenance is described. In this model, mice develop a lymphoid leukemia which remits when expression of BCL-2 is eliminated by treatment with doxycycline. To test the importance of BCL-2 in tumor maintenance in other tissue types, the conditional expression of BCL-2 will be extended to other tissues such as breast and melanocytes. Methods are described for the promotion of cancer in these models. When cancer develops, BCL-2 expression will be controlled by doxycycline to determine if BCL-2 is required for tumor maintenance. Finally, we will use the mitochondrial, cellular and mouse assays developed above to test the efficacy and specificity of BH3 mimetics of both small molecule and peptidomimetic origin. [unreadable] [unreadable]